Why Boeing Can Build A 737 In 9 Days (But The 747 Took Much Longer)

Boeing has been very fast at making the 737, maybe a little too fast, according to an NTSB investigation. As many as 52 Boeing 737s were leaving the assembly line every month by early 2019, thanks to production advances that shortened the assembly process to just nine days per plane. That’s a sharp contrast to earlier days of the 737, when seven planes were produced on a monthly basis.

But in March 2019, the FAA grounded the 737 MAX in response to two fatal crashes of the plane, one earlier that year and one in 2018. Production and sales would drop dramatically, but production would slowly start to rebound, that is, until January 5, 2024, when a 60-pound chunk of the fuselage blew off in the middle of Alaska Airlines flight 1262, depressurizing the entire cabin. This led to another temporary grounding, as well as another plummet in production and sales.

The January 5 incident was due to the 737 in question leaving the factory with an unsecured door plug. The NTSB would place at least part of the blame on the rushed production of the 737 MAX, and the company would be forced by the FAA to cap its monthly output to 38 planes. It would take until May 2025 for Boeing to build back up to those production numbers, though. But, thanks to its nine -day build process, the company plans to increase monthly production of the 737 MAX to 42 planes by the end of this year, with FAA approval. If all goes according to Boeing’s plan, production will ramp up to 57 planes per month by May 2027.

Boeing’s ambitious goals to ramp up production of the 737 may seem odd in view of the plane’s recent difficulties. But in spite of the 2018 and 2019 crashes, the 2024 Alaska Airlines incident, the FAA groundings, and the impact these events have had on sales, Boeing still has a massive backlog of over 4,000 orders to fill. Sales have begun to rebound, as well. Saudi-based AviLease, for example, recently ordered 30 Boeing 737 MAX planes. This follows an order by American Airlines for 85 737 MAX-10 planes, even though the MAX-10 isn’t even certified yet. In fact, the Boeing CEO claims that the 737 is sold out for the rest of the decade.

Then there are market pressures caused by competition. For instance, Airbus before the COVID-19 pandemic was able to produce over 60 of its A350 series planes on a monthly basis. It delivered 602 of these planes last year, averaging about 50 per month. Airbus plans to expand its production facilities and ramp up to 75 planes per month in 2026. Obviously, Boeing does not want to fall behind.

The financial bottom line is important, too. Boeing needs to keep manufacturing costs down on a plane like the 737 in order to stay competitive. The more time a plane spends on the production line, the more man-hours are involved. Furthermore, Boeing has to keep its customers and shareholders confident that it can deliver planes on a timely basis.

Building a jet in just nine days is no easy task. Boeing began production of the 737 in 1968 and, by 1998, it was producing a mere seven planes a month, as everything in the build process was pretty much done manually. That year it would increase monthly production of the 737 Next Generation to 14 planes. In 1999, Boeing radically changed the way this plane was manufactured, enabling the 737 to be built in 11 days. When Boeing moved on to the 737 MAX, the process was further streamlined to nine days.

Before this change, the 737 was assembled the way planes are traditionally built, which is called “garage-style”. With this method, the plane stays in one spot during the entire process, while the different teams come to it at various stages in the production timeline. But the large volume of orders placed for the Boeing 737 meant there needed to be a faster way to build it. So, the jet is now placed on a conveyor that moves it forward at a speed of 2 inches per minute from one assembly team to the next. In theory, if one team can’t finish its tasks before the plane has moved on, this is documented for the next team to finish. 

However, it is speculated that this is the part of Boeing’s 737 build process that broke down and resulted in the January 5, 2024, Alaskan Airlines incident. The unsecured door plug wasn’t documented, and thus the next team in the production didn’t know it needed to be secured. So, Boeing has modified the production to a “move-ready” process, whereby the plane is moved along the line only after the team that is currently working on it has finished critical tasks.

One of the ways that Boeing has streamlined the assembly process of the 737 is that it orders the fuselage pre-built, instead of building it in the plant. The hollow shell is delivered by train from a supplier in Wichita, Kansas. Once it arrives at the factory in Renton, Washington, the assembly can begin.

On days one through three, all the things that go inside the walls and panels, the wiring harnesses, plumbing, and insulation are installed, much the way a house is built after the frame and outer walls go up. At this stage, the avionic electronics are installed as well. Once these things are done, the hollow tube can be turned into an airplane.

On day four, the wings, also pre-built, are lifted by cranes and attached to the fuselage. Lasers are employed to ensure the precise alignment needed for the wings. The vertical tail fin is also attached, as well as the landing gear. At this point, the structure is looking like a real airplane, at least from the outside.

By day five, system functional tests are ready to be conducted. This is also when the horizontal stabilizer, or tailplane, is fitted. Most work shifts to the inside, where floor panels are laid and galleys and lavatories are installed. Final work on flight control wiring is completed, as well. On day six, the power is turned on. This allows the team to test the landing gear, making sure it retracts like it is supposed to. Other major subsystems are able to be tested as well. This is a very important point in the build process.

On day seven, the plane is able to stand on its own “feet”, so to speak. It is taken off of the assembly jacks and lowered on to its own wheels. This is also when the engines are mounted to the 737, and the engine wiring and fuel lines are installed. A lot of testing is done on day eight. Diagnostics are run on the flaps, slats, rudder, elevator and other flight controls. The cockpit systems are tested pretty rigorously, as well. The systems are specifically tested to see how they work together.

Final quality checks are done on day nine. The customer will conduct a walkthrough inspection while the plane is still in the factory. Once it passes all inspections, it is rolled out of the factory and on to a field for engine run-ups, taxi tests, and eventually a first flight. All that’s left is painting the plane, which will take a few days. But by the end of day nine, the 737 is fully assembled.

Since the Boeing 737 can be built so efficiently, it might seem odd that it would take so much longer to complete a 747, 43 days, to be precise. But did you know that a single Boeing 747 contains 150 miles of wiring and over 6 million parts overall? The sheer size and complexity of the widebody 747 is one reason building it would take so long to build. The 747 weighs 206 tons empty, over four times heavier than a 737, and its wingspan is nearly twice that of the 737. Each wing required 40,000 rivets, and moving and attaching these massive wings required the use of cranes, which would take some time.

The comparatively low sales volume of the 747 was another factor. During the 55 years it was in production, only 1,574 were built, compared to over 11,000 of the smaller 737s during that time. By the time production of the Boeing 747 was discontinued, only one plane was being built every two months, and the company was still keeping up with demand. It wasn’t practical to dramatically overhaul the manufacturing process for the 747, as it was for the 737. Simply put, Boeing never needed to build the 747 in nine days.

Boeing has certainly accomplished an impressive feat in its ability to build the 737 in just nine days on a consistent basis. But that speed in production hasn’t come without cost, as seen in the government findings for Alaska Airlines flight 1262. Still, Boeing is determined to keep rolling out 737s at an ever-increasing pace.